Nitrous oxide (N2O) gas is a minor constituent in the earth's atmosphere (˜300 ppbv), but the levels of N2O are rising due to human augmentation of the nitrogen cycle. N2O contributes to the destruction of the stratospheric ozone layer, increases the greenhouse effect of the atmosphere, and has a direct impact on human health. As a greenhouse gas (GHG), it is ˜300 times more destructive than carbon dioxide (CO2). Agricultural fertilizers are major sources of N2O. As the global population increases in the next few decades, the use of fertilizer will also increase to meet the demand for food.
The current technology used to quantify the emission of N2O from agricultural fields caused by fertilizer application is complex and very expensive. In most cases, it involves collecting emitted gases and analyzing them using GC or FTIR spectroscopy in the laboratory, or using very expensive laser spectroscopy methods including cryo-cooled Pb-salt tunable diode laser spectroscopy and cavity ring down spectroscopy. A highly sensitive technique to provide real-time analysis at ambient temperature is in demand. Further, N2O has weak absorption lines in the 1.55 μm wavelength band, which falls within the emission band of Erbium-doped fiber. So a real-time analyzer in the above band will not only be able to operate at room temperature but will can also be developed with available optical and electronic components used in the telecommunication industry, which will make the system compact and cost effective, and make it possible for the system to incorporate a multipoint sensor using fiber optic networking (see reference [1]).
A number of articles about N2O have identified the overtones of the characteristic absorption (fundamental) and the combinations of the overtones in the near infrared (NIR) region (1-2 μm) of the electromagnetic spectrum using Fourier transform absorption spectroscopy (see references [2-7]), cavity ring down spectroscopy (see references [8-10]) and intracavity laser absorption spectroscopy (see reference [11]).
U.S. Patent Application Publication No. 2015/0102240 discloses a gas detection system featuring an inner ring cavity fiber laser with a saturated absorption optical fiber. The system measures the laser intensity after passing through a gas cell situated in the closed-loop optical circuit of the ring cavity, and compares this against a previously stored reference value to detect changes in the concentration of a target gas found within the gas cell. While such techniques may be appropriate when the gas absorption or gas concentration is very high, the same solution is not suitable for detection of trace gases.
In the present application, Applicant presents for the first time a new technique based on intracavity fiber laser absorption spectroscopy (IFLAS), which can detect the weak absorption lines in the 1.52 μm band for N2O available from HITRAN (see reference [12]). The new technique uses the amplified spontaneous emission (ASE) present inside the laser cavity, and testing of a prototype has found the system capable of detecting and quantifying N2O gas at concentrations of around and below 100 ppbv [Reference 13].